Typical gas turbine engine fuel supply systems include a fuel source, such as a fuel tank, and a main fuel pump that receives fuel drawn from the fuel source and delivers pressurized fuel to the fuel manifolds in the engine combustor via a main supply line. The main fuel pump is typically implemented using a positive displacement pump that is driven directly by the engine gearbox. Thus, the fuel flow supplied by the main fuel pump is proportional to engine speed.
The fixed displacement of a main fuel pump is typically sized to produce the fuel flow that is needed to run the engine at a max demand case, which is typically during engine start-up, where engine speed is relatively low, or during takeoff, where fuel demand is relatively high. As such, at other operating conditions, such as idle or high altitude cruise, the main fuel pump supplies much more fuel than the engine needs. In many instances, the excess fuel is recirculated back to the low pressure pump inlet or some other low pressure point upstream of the main fuel pump.
The known fuel supply systems described above generally operate safely and robustly, but can exhibit certain drawbacks. For example, recirculating the excess fuel from the main fuel pump wastes energy by generating heat, and can lead to undesirably high fuel temperatures. Moreover, developments in turbine engine combustors are driving up the required fuel pressures, and include multiple, separate combustor zones. The increased fuel pressures are, in many instances, beyond the level that can be produced by a single positive displacement pump (gear or vane). In addition multiple fuel metering circuits are needed to supply fuel to the separate combustor zones, a departure from the known fuel supply systems in which the unneeded fuel from a single metering circuit is recirculated back to the low pressure pump inlet.
Hence, there is a need for a gas turbine engine fuel supply system that wastes less energy than presently known systems, supplies higher fuel pressures than presently known systems, and does so to separate combustor zones. The present invention meets at least these needs.